Guide – Resistivity

The electrical resistance of the Ground is almost entirely dependant upon the amount and distribution of moisture within it. Buried remains affect this distribution and can be detected with instruments. Stone, for example, is more moisture resistant than a clay subsoil or the filling of a ditch. These resistivity differences can be detected and when overlaid on a map will often give a plan of buried remains.

Resistivity survey methods have been used to detect both natural and archaeological features since the techniques discovery in 1946.

When one encounters a soil resistivity survey in progress, several thoughts spring to mind. How can objects deep under the ground be detected by electrodes inserted only a few centimetres? And why are four (sometimes five) electrode probes needed? Do differing types of soil affect the precision of the readings?

This guide aims to provide sufficient background information to allow the lay person to understand the basic principles of resistivity survey for archaeology.

First Principles

Those who remember physics lessons in school may remember that when an electrical voltage is applied between the ends of an electrical conductor such as wire, a current flows through it; the size of the current depending on the resistance of the conductor. The symbol for resistance is R, measured in ohms (often represented by the Greek letter omega).

Water, in its natural state, is an insulator. However, with a little salt added, it soon allows current to flow. Chemicals which have this effect on water are known as electrolytes. The resistance of soil is almost entirely dependant on its water content and the electrolyte “mix” it contains. Most of its other components, such as stone, are largely insulators. Buried wood generally tends to attract water and so reduces the resistance in that area.

Resistivity is a uniform measure which allows the resistance of different substances to be compared. It is defined as the resistance of a cubic meter of material when a 1 volt charge is applied between the two opposite faces of the cube. The unit of resistivity is the ohm metre, its symbol is the Greek letter rho.

The resistivity of soil can vary from 1-10 ohm-metres; porous rocks 100-1,000 ohm-metres and non porous rocks anything from 10k to 10m ohm-metres.

Contrary to how it may first appear, current does not flow through soil as a direct path. Current flowing between two electrodes in soil will spread out into a myriad of paths rather like the force bands surrounding a magnet. The total resistance is a sum of the resistance offered down each path. It can be seen therefore that a ditch cut through a rock base will show lower resistance in surface measurements than the natural soil and rock layer around it. In fact, the lower the resistance the deeper the current will travel – due to the like charged particles repelling each other, thus causing a wider spread of current.

Technically, resistivity measurements could be made using a household resistance meter. A calculation would be required to work out the resistivity and indeed some of the first resistivity devices used by archaeologists were based on an electrician’s “Megger” – a device normally used to certify domestic electricity circuits. However, due to design limitations these devices are generally unsuitable for soil resistivity measurement.

Measuring Soil Resistance

Measuring the resistance of soil presents us with problems. The electrodes applying the current have a small contact area compared to the volume of ground to be measured. At its surface soil tends to be dry, thus providing a poor contact medium. These effects create a much higher resistance in the immediate area of the electrodes, which would tend to cancel out any reading from the ground in between.

The solution to this problem was found by creating a probe with four electrodes. Known as the Wenner system, these are placed at equal distance in a line – the outer two apply the current, the inner two measure the voltage of the ground. These two measurements – voltage and current – are used to calculate the ground resistance (R=V/I).

In addition to this a high impedance measuring circuit helps take into account variations in surface contact conditions. An AC circuit is used as DC current would effectively turn the soil into a battery and mess up the readings. A further refinement uses a phase-sensitive rectifier to cancel out other interference.

Types of instruments

Manual Balance Instruments

These early instruments used an on-board dial to allow the resistance to be matched and noted; often the probes were pushed in individually. In some cases a rotary switch and a five probe design allowed a measurement to be taken each time a single probe was moved. Often two skilled operators were required.

Automatic Instruments

In general, manual balance systems have been consigned to the past, thanks to electronic devices such as data loggers which automatically sample and store the measurements, and the creation of a probe ‘cradle’ which allows an individual to survey a field at near walking speed.

Data Loggers

With a data logger, every time the probe cradle is inserted into the soil, a button is pressed to take a sample. The data logger – an electronic device attached to the cradle – takes the resistance readings and stores them in sequence.

Later, either on site or in the office, these samples can be plotted against a map to provide a clear picture of the resistivity changes of the subsoil, often giving feature markings so clear that little confirmation excavation is needed.

Laptop Computers and Beyond

The advent of the laptop and sub-laptop computer, together with the ability to provide data logging and sampling onboard, will ultimately create a cradle capable of giving a “live” display of the underground resistance topology. The technology for this advance exists currently, but will need a little development before it can be realised.

Resistivity for Archaeology

A feature of high resistance buried in the ground will cause the resistance of the overall local area to increase, this is known as a “positive anomaly”. Conversely, a feature such as a rock cut ditch will lower the overall resistance and is known as a “negative anomaly”.

In the early days it was assumed that based on the above, features such as stone foundations and walls would always give high resistivity readings and therefore be positive anomalies. However experience has shown that whilst this can be the case, often other factors such as the features geometry, associated deposits, soil moisture content and electrode configuration can cause complications to this rule.

However, a significant amount of research effort has created a range of designs which offer a robust and reliable surveying tool.

Electrode Configurations

The first experiments with soil resistance were carried out by Frank Wenner in 1916. His original four electrodes in a line configuration with two current electrodes to apply power (C1,C2) and two potential electrodes to measure resistance (P1,P2) has been adopted and modified by archaeologists based on the results of extensive testing.

During testing, it became clear that for some features the Wenner and related probe configurations were not effective at detecting some types of underground features. Narrow features were found to show double or even treble readings.

As a result of this a wide range of electrode configurations have been evaluated giving archaeologists the option of a range of electrode configurations to suit the ground and type of feature.

The Wenner configuration is still a commonly used configuration, as it offers good all round functionality for most types of submerged features. The wenner configuration can sometimes exaggerate the width of the anomaly and is susceptible to misinterpret some high resistance features.

C1 P1 P2 C2

Wenner electrode configuration

For very shallow features, the Double Dipole configuration has been shown to give particularly good results, this configuration, also known as the Wenner beta configuration is created by taking the Wenner configuration and swapping on current electrode for a potential electrode.

C1 C2 P2 P1

Double dipole electrode configuration

A more recent development, the Twin Electrode configuration sees the Wenner design cut in half and provides for a half size cradle as well as almost eliminating some of its inaccuracies. With the twin electrode configuration, two probes are fixed at a static point to one side of the test area. The other two probes are attached via a long lead and are moved around the survey site. This design helps to eliminate the exaggeration of high resistance features.

C1 C1 – – – – – – – – – – – – – – – – – – – -C2 P2

Fixed Mobile probe

Twin electrode configuration

One other electrode configuration worthy of mention is the Square Array, this was developed as a solution to the poor response given by the Wenner configuration to small buried objects. With this configuration the probe looks like a small table and tends to be used in more specialist circumstances.

C1 C2

P1 P2

Square Array configuration

The Impact of Soil

Now that we have a better understanding of the relationship between the buried feature and the types of probes used for resistivity, it is important to understand the effect different soils and moisture levels with readings.

The structure of soil

Generally, when we are talking about soil, we are considering several different factors, each of which are inter-related. Firstly, the top layer of soil is usually a loam type material with varying amounts of other materials either due to natural deposition or related to the natural bedrock. The actual resistivity of this soil is a combination of this soil composition and the retained moinsture at the time of the survey. The level of moisture retained by soil is a result of that particular soils natural drainage, and the drainage provided by the underlying bedrock.

In some times of the year, the soil effectively becomes waterlogged and this will result in many features being hidden by the overall low resistance of the soil

The following section illustrates the impact of bedrock on a features resistivity and discusses the impact of rainfall in order to identify the likely results for a particular feature as well as proposing the best time of year to survey for each bedrock type.

Bedrock type

Feature

Dimensions

Anomaly type

“Season”

Best

Sandstone

Ditches

W1-4m D3m

Low

Jun-Sept

July

Clay

Rubble Wall

W10m D1m

High

Jun-Nov

Sept

Limestone

Stn Coffin

W.5m D1.5m

High

Jul-Oct

Oct

Chalk

Ditch

W18m D6m

Low

Dec-Jun

Mar-Apr

Chalk

Ditch

W2.5m D1m

High

Jul-Nov

Sept

Chalk

Ditch

W6m D2m

Low/High

Dec-June

Mar-Apr

The Best “Season”

The above table gives example responses for types of anomaly and time of year. The first point to note is that features provide differing levels of response throughout the year. The main reason for this is the amount of rain held in the soil. As soil becomes soaked its resistance lowers until the readings from many features become “swamped” or hidden by this low resistance (low resistance features will disappear once the surrounding soil reaches the same resistance. Also hi resistance features may go by unnoticed when surrounded by very low resistance soil).

The amount of moisture retained by the soil is largely dependent on two things – amount of rainfall and drainage. Rainfall is typically seasonal, thus over the winter months many sites became waterlogged and unreadable.

The drainage for a particular site is dependent on many localised factors such as slope of the land. The underlying bedrock however commonly plays a significant role in determining the soils water content. An impervious rock such as sandstone will typically retain moisture for longer and therefore the resistivity “season” is shorter. By the same reasoning each local area will have a best time to survey depending on the local drainage and recent rainfall.

The size and nature of the buried feature also has an impact on the definition shown by a resistivity survey. Stone tends to have a higher resistance than the low resistance soil and even with relatively high resistance bedrock such as clay. Ditches generally always show a low resistance, the deeper the ditch the lower the resistance since deep ditches cut through the bedrock, lowering the depth of the low resistance fill.

Chalk can give conflicting results due to its structure. When it is dry, small ditches will tend to show a low resistance reading. However, chalk can act like a sponge when waterlogged, changing its resistance “form” from damp chalk to that of chalky water. In these conditions the bedrock becomes very low resistance and the ditch reads as a high resistance anomaly. Furthermore it has been found that in some cases due to the local water table the anomaly can fluctuate between high – neutral – low readings throughout the year and it is with this in mind that in chalk areas two resistivity surveys approximately. six months apart are recommended (spring and autumn).

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